A central characteristic of primate lentiviruses that distinguishes them from onco-retroviruses is their ability to infect non-dividing cells. As a result, primate lentiviruses such as HIV-1 can establish a productive infection in terminally differentiated cells such as macrophages and microglia. While most of the research into understanding early events in retrovirus replication have focused on steps prior to nuclear localization of viral cDNA, events in viral replication subsequent to engagement of the nuclear envelope and prior to integration within chromatin have received very little attention. As a result, viral and cellular proteins that regulate this step in viral replication have not been well characterized. Studies conducted in the previous funding period have led to the identification of nuclear envelope proteins that regulate a step in the viral life cycle subsequent to nuclear entry of viral cDNA but prior to integration of viral cDNA into host cell chromatin. In addition, we have evidence that HIV-1 and MLV, viruses with very different host cell cycle requirements, have overlapping requirements for components of the nuclear envelope during infection of the cell. In the next funding period we will define how proteins of the nuclear envelope regulate the infectivity of HIV-1 (and MLV) and will extend our analysis to identify cellular proteins that regulate the access of HIV-1 cDNA to the nucleus. Specifically we propose to: Aim 1: Identify cellular factors that mediate interaction of HIV-1 with the nuclear envelope and how this interaction regulates viral infectivity. Aim 2: Characterize viral determinants that mediate interaction of HIV-1 with components of the nuclear envelope. Aim 3: Survey proteins of the nuclear envelope by RNA interference to identify cellular factors that regulate transit of HIV-1 cDNA through the nuclear envelope. Understanding the mechanism through which nuclear envelope proteins regulate HIV-1 infection may ultimately point to novel therapeutic strategies that truncate the viral replication cycle prior to establishment of the provirus.